A new and more sensitive cell line, showing contact-inhibition during confluency, will be employed so that quantitative studies on xenotropic virus titers from reproductive tissues of mice will be more readily isolated, quantitated and characterized. The adaptation of the erythroleukemia cells from the conventional in vitro suspension culture system to suitable plating and cloning systems will be made as a prerequisite for these studies. With the availability of this mass culture technique it then becomes possible to induce and select auxotropic mutants.